The present invention relates to a method and a structure for connecting a plurality of cells in series for forming a battery module having a desired output voltage.
In order to form a battery module having a desired output voltage by connecting a plurality of cells in series, it is known in the art to employ connectors. Generally, a cell for constituting a battery module includes a negative electrode in the form of metallic cell case which covers the cell entirely except for one end surface thereof. The opening on one end of the cell case is closed with a sealing assembly including a positive electrode plate made of metal. When connecting two cells in series, a connector is interposed between the positive electrode plate of one cell and the cell case constituting the negative electrode of the other cell, and electrically connected at predetermined locations to both of them. The connector is spot-welded to the cells, and therefore it connects the two cells not only electrically but also couples them mechanically together.
Spot welding usually includes applying electric current across a pair of welding electrodes which tightly hold a portion to be welded, so as to melt the portion held between the welding electrodes. In welding two cells together with a connector, however, the connector is interposed between the cells and cannot be held between the pair of welding electrodes, as a result of which electric current flows through the surfaces of the connector or the electrode plate across the welding electrodes, and in cases where the connector is thick or has a low specific resistance, the welding becomes difficult. That is, in accordance with the prior art method of connecting cells, since materials having a large thickness or a low specific resistance cannot be used for the connector, the electric resistance in the joint between two cells is large, and accordingly the loss at the joints where large current flows becomes large. Moreover, the mechanical strength of the connector is low, making the mechanical connection between adjacent cells low.
The inventors of the present application have previously proposed an improved structure for connecting cells and a welding method for reducing electrical connection resistance while enhancing mechanical connection strength, whereby the problems found in prior art are resolved. (Japanese Laid-open Patent Application No. 10-106533). FIG. 5 is a longitudinal cross sectional view showing this cell connecting structure. A connector 1 includes a flat bottom plate 3 abutted on a positive electrode plate 2 of one cell B1, and a cylindrical wall 7 oupled to a cell case 4 of the other cell B2 which doubles as the negative electrode thereof. A plurality of projections 8 and 9 for the welding purpose are formed each in circles on the cylindrical wall 7 and on the bottom plate 3. The drawing shows the cells before welding. The connector 1 is welded to the electrode plate 2 of one cell B1 through the projections 9 and joined thereto, while it is welded to the cell case 4 of the other cell B2 through the projections 8 and joined thereto.
Projection welding involves generation of heat by local concentration of electric current at the joints between two members to be welded together contacting each other in a very small area such as the projections 8, 9, where the electric resistance becomes largest. The contacting area melts by the heat and the two members are welded together. The portions to be welded need not be held tightly between the welding electrodes as in the spot welding. The connector 1 in the above-described connecting structure is welded respectively to the two cells B1, B2 by such projection welding, and therefore the choice of the material for the connector 1 need not depend on its thickness or specific resistance, and also, the welding strength can be improved. Moreover, since the electrode plate 2 and the cell case 4 are welded to the connector 1 at a plurality of locations on matching circles, the electric current flows the shortest distance through the joint between the cells. Thus the cells are coupled with high mechanical strength as well as with low electric resistance.
The cell connecting structure offers such advantages as described above, but still has problems to be resolved in practical application. That is, the projection welding in accordance with the above-described method includes at least two welding steps, one for welding the bottom plate 3 of the connector 1 to the electrode plate 2 of one cell B1, and one for welding the cylindrical wall 7 of the connector 1 to the cell case 4 of the other cell B2.
As the number of the cells to be connected increases, so does the number of the connector 1. The connector 1 has a considerably complex structure: It has a circular shape having a stepped bottom and a cylindrical wall projecting over the outside of the cell case 4 of cell B2 for detouring the gasket 10 of cell B1, and has an opening 12 in its center at the bottom for passing the cap-shaped positive electrode terminal 11 of cell B1 therethrough. Furthermore, although not shown, the cylindrical wall 7 of the connector 1 is formed with cut-outs for providing resiliency so that it makes tight engagement with the cell case 4 of cell B2 when coupled thereto, and the bottom plate 3 is also formed with cut-outs for preventing wasted current during the projection welding. Thus the connecting structure described above has high costs because of the large numbers of welding processes and components such as the connector 1, which has such a complex shape as described above and cannot be obtained at low cost.
Another problem is that, since the cylindrical wall 7 of the connector 1 is coupled over the cell case 4, the resultant battery module wherein a plurality of cells are connected has protruding portions at each joint of adjacent cells because of the increased outer diameter, leading to the problem of dimensional precision of an outer tube for covering the battery module. Furthermore, electric resistance at the joint between adjacent cells becomes large by the length of the electric current path passing through the connector 1.
In view of the problems in prior art as described above, it is an object of the present invention to provide a method and structure for connecting cells, whereby a reduction in cost is achieved, protruding portions are eliminated from the cell case of each cell, and the electric resistance at the joints between adjacent cells is reduced.
In order to achieve the above object, the present invention provides a cell connecting structure for coupling together two adjacent cells in a battery module wherein a plurality of cells are arranged coaxially in series and connected electrically. Each of the cells includes a cylindrical cell case having an open end and a bottom surface on the other end doubling as one electrode of the cells, and a sealing assembly for sealing the open end of the cylindrical cell case. The cell connecting structure of the invention includes a cell case of a first cell, and a sealing assembly, having a specific structure, of a second cell. The sealing assembly includes an electrode plate formed with a concavity in a central portion thereof, a valve body housed in the concavity, a valve body holding plate welded to an upper peripheral edge of the concavity for sealing the concavity, a connection electrode formed in an annular shape and protruded outwards in a direction along the axis of the cells at a location near outer peripheral edge of the electrode plate, and a plurality of projections formed on the connection electrode. The plurality of projections on the connecting electrode of the second cell are directly welded to the bottom surface of the cell case of a second cell.
According to the cell connecting structure of the invention, two adjacent cells among the plurality of cells arranged in series are connected in a single welding process step, wherein the connection electrode of one cell is directly welded to the cell case of the other cell without using a conventional connector. Accordingly, a considerable reduction in cost is achieved in view of the reduced number of welding process steps and the fewer number of parts such as connectors. Also, the two cells are coupled together at a location near the outer periphery of the cells, and therefore the bending strength between the cells is improved and they are stably connected with sufficient mechanical strength. A battery module formed by connecting a number of cells in accordance with such method can be housed precisely within a simple, cylindrical outer tube, because it has no protruding parts such as the conventional connector. Furthermore, the cell connecting structure enables electric current to pass through the shortest possible path between two cells, through the connection electrode that doubles as one electrode for one cell and the cell case that doubles as the other electrode for the other cell, and electric resistance is reduced accordingly.
The dimensional precision of the space for accommodating a valve body is improved as compared to the prior art sealing assembly structure, wherein such space was defined by the shape of a cap-shaped positive electrode terminal. The projections for the welding are formed on the connection electrode that protrudes upwards, and therefore the height of the projections can be freely set irrespective of the valve body accommodating space. Moreover, the connection electrode is formed in a ring-like shape along the periphery of the cell case, whereby the bending strength between the connected cells is enhanced.
In order to achieve the above object, the present invention provides a cell connecting method for coupling together two adjacent cells by welding, in a battery module wherein a plurality of cells are connected electrically in series. Each of the cells includes a cylindrical cell case having an open end and a bottom surface on the other end doubling as one electrode of the cells and a sealing assembly for sealing the open end of the cylindrical cell case. The cell connecting method includes the steps of:
forming a connection electrode in an annular shape protruded outwards in a direction of axis of the cells at a location near outer peripheral edge of the sealing assembly of the cells;
providing a plurality of projections on the connection electrode;
placing the plurality of projections of a first cell in contact with the bottom surface of the cell case of a second cell;
abutting a first welding electrode on a peripheral side face of the connection electrode of the first cell;
abutting a second welding electrode on a peripheral side face of the cell case of the second cell; and
applying voltage across the first and second welding electrodes for welding the plurality of projections on the connection electrode of the first cell with the bottom surface of the cell case of the second cell.
Preferably, the first welding electrode abutted on the peripheral side face of the connection electrode of the first cell includes a planar insert having a width such as to allow itself to be inserted into a gap between a top end of the first cell and the bottom surface of the second cell, the planar insert being formed with an indentation conforming to the external contour of the connection electrode of the first cell.
Since there has not been devised a welding electrode having such planar insert as mentioned above, the prior art method involved at least two welding process steps as discussed with respect to the background art, in which the bottom plate 3 of a connector 1 was welded to the electrode plate 2 of one cell B1, after which two cells B1, B2 are arranged in series in close contact with each other, and then the welding electrode was abutted on the outer side of the cylindrical wall 7 of the connector 1 for welding the cylindrical wall 7 to the cell case 4 of the cell B2.
According to the present invention, because the welding electrode includes a planar insert having a width such as to allow itself to be inserted into a gap between the top of the first cell and the bottom of the second cell, the welding electrode can be inserted between two adjacent cells. In this way, the connection electrode of one cell is directly connected to the bottom of the cell case of the other cell in a single welding process step, and a considerable reduction in cost is achieved in view of the reduced number of welding process steps and the fewer number of parts such as connectors. Moreover, since the planar insert is formed with a semi-circular indentation at the distal end thereof which conforms to the outer shape of the connection electrode, electric current flows uniformly through the connection electrode via the planar insert, whereby the welding is performed favorably.
The planar insert is coated with a nonconductive layer on an upper face and a lower face thereof, whereby the welding electrode that is inserted into a small gap between two cells is prevented from contacting with either one of the cells located closely above or below the welding electrode and causing a short-circuit.
The plurality of projections for the welding purpose may be provided on the cell case of the cell instead of on the connection electrode on the sealing assembly of the cell, for achieving the same or similar benefits.
While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.